Effects of Mountain Glaciation

The three diagrams here show a mountain landscape before a glacial advance, at
the maximum extent of the glaciers, and after the glaciers have melted.

Labeled Diagram

Glaciers don't cut brand new valleys. Instead, they flow down existing river
valleys. In the landscape below, ice would begin accumulating in the shade at
the tops of the highest valleys, and as it thickened, it would begin flowing
downhill.

Since ice flows like a very thick fluid, it can't get into tight spots and
erode like water can. Instead it simply planes off whatever is in the way,
creating the U-shape so typical of glacial valleys. As the ice flows down from
its source, it plucks rock away from the head of the valley, creating a
bowl-shaped basin called a cirque. Some of the debris carried by the
glacier is deposited along the sides to form a lateral moraine. When two
glaciers join, their lateral moraines form a streak of debris in the middle of
the glacier, called a medial moraine. Debris dumped at the end of the
glacier is called a terminal moraine.

When the ice retreats, the former glacial valleys are easily identified by
their U-shapes and cirques, and the margins of the glacier are marked by
moraines. Medial moraines occur whenever two active glaciers join, but are
rarely seen after the ice melts because melt water generally erodes them.
Valleys that once joined the original river valley are now chopped off by the
ice and end high above the valley floor. These hanging valleys account
for many of the world's highest waterfalls.